The intensive introduction of advanced composites or composite materials in primary aircraft structures has become one of the priority objectives in the design and manufacture of a new generation of aircraft because of the possibilities they provide for structural optimization.
As is well known, the manufacture of composite parts made from polymeric fibrous reinforcements and resins for aircraft structures involves its curing within a vacuum bag under certain conditions of temperature and pressure. It is therefore essential that no pressure loss occur in the vacuum bag during curing processes.
Vacuum bags used for manufacturing composite parts consist of different types of plastic materials depending on the maximum temperature of use and their elongation capacity until their rupture. The temperature range can be between 420° and 120° and the elongation capacity can reach 500%. Different materials such as nylon, polyolefin, polyamide or thermoplastic elastomers in one or more layers are used.
As these materials are susceptible to breakage, the manufacturing processes of composite materials must necessarily consider the use of leak detection methods in vacuum bags.
The existence of such leaks can be easily detected through measurements of pressure within the vacuum bag but their location can be very difficult particularly in the case of large bags using known methods and in particular ultrasonic detectors. The method probably most used in industry for locating leaks in vacuum bags is based on the use of ultrasound, and in fact, manufacturers of vacuum bags often include in their catalogs ultrasonic detectors that convert the ultrasound produced by pressure losses in audible sound that is amplified as closer is to the detector.
There are other proposals, such as that described in WO 2008/088435 A1, which is based on the incorporation into the vacuum bag of a film sensitive to oxygen that changes appearance when a leak occurs.
The use of thermograph equipment (particularly IR cameras) for detecting leaks is also known.
Another method of detecting leaks in tools includes the use of a helium detector wherein a helium emitter is moved along the tool when placed under vacuum. When helium is drawn into a leak, the detector is activated in order to indicate the location of the leak.
These detection techniques have significant costs and do not adapt equally to various sizes of vacuum bags that are used particularly in the aircraft industry. Ultrasound detectors require a complete inspection of the part approaching the ultrasonic detector to the areas of the part susceptible to have leaks, which entails significant costs and limits its use in large pieces. The incorporation into the vacuum bag of a film sensitive to oxygen significantly increases its cost.
In any case, there is a need of inspection methods and systems for detecting leaks that produce pressure loss of small entity in vacuum bag assemblies for manufacturing large composite parts, particularly in the case of manufacturing methods where the vacuum bag covers directly all the components of the composite part (called “direct bag” methods).